Glycogen synthase sensitivity to insulin and glucose-6-phosphate is mediated by both NH2- and COOH-terminal phosphorylation sites

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54 Scopus citations


In skeletal muscle, insulin activates glycogen synthase by reducing phosphorylation at both NH2- and COOh-terminal sites of the enzyme and by elevating the levels of glucose-6-phosphate, an allosteric activator of glycogen synthase. To study the mechanism of regulation of glycogen synthase by insulin and glucose-6-phosphate, we generated stable Rat-1 fibroblast clones expressing rabbit muscle glycogen synthase with Ser → Ala substitutions at key phosphorylation sites. We found that 1) elimination of the phosphorylation of either NH2- or COOH-terminal sites did not abolish insulin stimulation of glycogen synthase; 2) mutations at both Ser-7 and Ser- 640 were necessary to bypass insulin activation; 3) mutation at Ser-7, coupled with the disruption of the motif for recognition by glycogen synthase kinase-3 (GSK-3), did not eliminate the insulin effect; and 4) mutation of either Ser-7 or Ser-640 increased the sensitivity of glycogen synthase to glucose 6-phosphate >10-fold. We conclude that Ser-7 and Ser-640 are both involved in mediating the response of glycogen synthase to insulin and activation by glucose 6-phosphate. In Rat-1 fibroblasts, GSK-3 action is not essential for glycogen synthase activation by insulin, and GSK3-independent mechanisms also operate.

Original languageEnglish (US)
Pages (from-to)1096-1100
Number of pages5
Issue number7
StatePublished - Jul 2000

ASJC Scopus subject areas

  • Internal Medicine
  • Endocrinology, Diabetes and Metabolism

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